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Genetic traces from a pair of 1,500-year-old teeth have shown how bubonic plague pandemics can emerge independently.

An international research team, including Professor Edward Holmes of the University of Sydney’s School of Biological Sciences, used the tooth fragments to reconstruct the ancient genome.

They found that the 6th century Plague of Justinian, which killed half of the world’s population, was the result of a different version of the pathogen that caused the Black Death more than 700 years later. It is now thought to be extinct.

The study indicates that new plagues may emerge in the future, though the existence of modern antibiotics will likely limit their spread.